Sorry for the confusion...
I just checked what happens on my system (fetchmail -> sendmail -> procmail) and the error reported to the logs by sendmail and fetchmail are:
"553 5.1.8 Domain of sender address firstname.lastname@example.org
does not exist"
i.e. not a 451 error.
The fetchmail man page lists error code 553 as the only case where it will delete the message without delivery (or bounce).
Looks like the problem could be with the sendmail configuration. Check you have the latest software package and check that sendmail.cf is up to date, possibly built from sendmail.mc.
The error is actually generated in the "checkmail" ruleset. Search the file for the "Domain of sender address" string:
$ grep 'Domain of sender address' /etc/mail/sendmail.cf
R<TEMP> $* $#error $@ 4.1.8 $: "451 Domain of sender address " $&f " does not resolve"
R<PERM> $* $#error $@ 5.1.8 $: "553 Domain of sender address " $&f " does not exist"
The first bit is a test for temporary or permanent DNS lookup failures. So if your sendmail.cf matches mine, the problem could be with DNS lookups. Are you using any DNS blacklists?
Check your /etc/resolv.conf for non-functioning nameserver entries.
Hope this helps,